Continuous steam generator

ABSTRACT

A continuous steam generator is provided. The continuous steam generator includes a number of burners for fossil fuels, the outside wall thereof being fully or partially formed from steam generator tubes welded together in a gas-tight manner. The burners are arranged in a combustion chamber, and a vertical gas duct is mounted downstream of the combustion chamber above a horizontal gas duct on the hot gas side. A first part of the steam generating tubes forms a system of evaporation tubes mounted upstream of a water separator system, on the flow medium side, and a second side, and a second part of the steam generating tubes forms a system of superheater tubes mounted downstream of the water separator system on the flow medium side. Superheater tubes adjacent and parallel to evaporation tubes are mounted directly downstream of the water separator system on the flow medium side.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2009/061468, filed Sep. 4, 2009 and claims the benefitthereof. The International Application claims the benefits of EuropeanPatent Office application No. 08015871.0 EP filed Sep. 9, 2008. All ofthe applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a once-through (“continuous”) steam generatorcomprising a number of burners for fossil fuel, the surrounding wallthereof being completely or partially formed from steam generator tubeswelded together in a gas-tight manner. Said burners are disposed in acombustion chamber downstream of which a vertical gas duct is mountedabove a horizontal gas duct on the hot gas side, a first part of thesteam generator tubes being implemented as a system of evaporator tubesmounted upstream of a moisture separation system on the flow mediumside, and a second part of the steam generator tubes being implementedas a system of superheater tubes mounted downstream of the moistureseparation system on the flow medium side.

BACKGROUND OF INVENTION

In a fossil fired steam generator, the energy of a fossil fuel is usedto produce superheated steam which in a power plant, for example, canthen supplied to a steam turbine for power generation. Particularly atthe steam temperatures and pressures prevalent in a power plantenvironment, steam generators are normally implemented as water tubeboilers, i.e. the water supplied flows in a number of tubes which absorbenergy in the form of radiant heat of the burner flames and/or byconvection from the flue gas produced during combustion.

In the region of the burners, the steam generator tubes are usuallywelded together in a gas-tight manner to form the combustion chamberwall. In other areas downstream of the combustion chamber on the fluegas side, steam generator tubes disposed in the waste gas duct can alsobe provided.

Fossil fired steam generators can be categorized on the basis of a largenumber of criteria: based on the flow direction of the gas stream, steamgenerators can be subdivided, for example, into vertical and horizontaltypes. In the case of fossil fired steam generators of vertical design,a distinction is usually drawn between single-pass and two-pass boilers.

In the case of a single-pass or tower boiler, the flue gas produced bycombustion in the combustion chamber always flows vertically upward. Allthe heating surfaces disposed in the flue gas duct are located above thecombustion chamber. Tower boilers offer a comparatively simple designand simple control of the stresses produced by the thermal expansion ofthe tubes. In addition, all the heating surfaces of the evaporator tubesdisposed in the flue gas duct are horizontal and can therefore becompletely dewatered, which may be desirable in frost-proneenvironments.

In the case of the two-pass boiler, a horizontal gas duct leading into avertical gas duct is mounted in an upper region downstream of thecombustion chamber on the flue gas side. In said second vertical gasduct, the gas usually flows vertically from top to bottom. Therefore, inthe two-pass boiler, multiple flow baffling of the flue gas takes place.Advantages of this design are, for example, the lower installed heightand the resulting reduced manufacturing costs.

Steam generators may also be designed as natural circulation, forcedcirculation or once-through steam generators. In a once-through steamgenerator, the heating of a number of evaporator tubes results incomplete evaporation of the flow medium in the evaporator tubes in onepass. Once evaporated, the flow medium—usually water—is fed tosuperheater tubes downstream of the evaporator tubes where it issuperheated. Strictly speaking, this description is valid only atpartial loads with subcritical pressure of water (P_(Kri)≈221 bar) inthe evaporator—at which there is no temperature at which water and steamcan be present simultaneously and therefore also no phase separation ispossible. However, for the sake of clarity, this representation will beused consistently in the following description.

The position of the evaporation end point, i.e. the location at whichthe water content of the flow is completely evaporated, is variable anddependent on the operating mode. During full load operation of aonce-through steam generator of this kind, the evaporation end point is,for example, in an end region of the evaporator tubes, so that thesuperheating of the evaporated flow medium begins even in the evaporatortubes.

In contrast to a natural or forced circulation steam generator, aonce-through steam generator is not subject to pressure limiting, sothat it can be designed for main steam pressures well above the criticalpressure of water.

During light load operation or at startup, a once-through steamgenerator of this kind is usually operated with a minimum flow of flowmedium in the evaporator tubes in order to ensure reliable cooling ofthe evaporator tubes. For this purpose, particularly at low loads ofe.g. less than 40% of the design load, the pure mass flow through theevaporator is usually no longer sufficient to cool the evaporator tubes,so that an additional throughput of flow medium is superimposed in acirculating manner on the flow medium passing through the evaporator.The operatively provided minimum flow of flow medium in the evaporatortubes is therefore not completely evaporated in the evaporator tubesduring startup or light load operation, so that unevaporated flowmedium, in particular a water-steam mixture, is still present at the endof the evaporator tubes during such an operating mode.

However, as the superheater tubes mounted downstream of the evaporatortubes of the once-through steam generator and usually not receiving flowmedium until it has flowed through the combustion chamber walls are notdesigned for a flow of unevaporated flow medium, once-through steamgenerators are generally designed such that water is reliably preventedfrom entering the superheater tubes even during startup or light loadoperation. To achieve this, the evaporator tubes are normally connectedto the superheater tubes mounted downstream thereof via a moistureseparation system. The moisture separator is used to separate thewater-steam mixture exiting the evaporator tubes during startup or lightload operation into water and steam. The steam is fed to the superheatertubes mounted downstream of the moisture separator, whereas theseparated water is returned to the evaporator tubes e.g. via acirculating pump or can be drained off via a flash tank.

However, particularly in startup mode, the above mentioned conceptcauses high temperature differences between evaporator tubes andsuperheater tubes: during cold starting, as yet unevaporated flow mediumflows in the evaporator tubes at saturation temperature, while steam athigher temperature is still present in the superheater tubes. During hotstarting, on the other hand, the evaporator tubes are filled with coldfeedwater, while the superheater tubes are still at operatingtemperature level. This can result in overloading and damage of thematerials due to the differential thermal expansion.

SUMMARY OF INVENTION

The object of the invention is therefore to specify a once-through steamgenerator of the above mentioned type requiring comparatively low repaircosts and having a comparatively long service life.

This object is achieved according to the invention by mountingsuperheater tubes in parallel contiguity with evaporator tubesimmediately downstream of the moisture separation system on the flowmedium side.

The invention is based on the idea that it would be possible to reducerepair costs and increase the service life of the once-through steamgenerator if damage caused by differential thermal expansion ofwelded-together steam generator tubes could be minimized. Thedifferential expansion is the result of high temperature differencesbetween the steam generator tubes. Said temperature differences arecaused by differential cooling of the steam generator tubes anddifferent temperatures of the flow medium flowing therein and thereforeoccur in particular at the interface between welded-together evaporatorand superheater tubes, as these exhibit a different throughput of flowmedium with different temperatures through the intervening moistureseparation system particularly during cold and hot starting.

Particularly in the case of once-through steam generators of thetwo-pass type, the design means that an interface betweenparallel-welded evaporator and superheater tubes is typical. In order tominimize as far as possible the temperature differences betweenevaporator and superheater tubes, the steam temperature in thesuperheater tubes welded parallel with the evaporator tubes must beminimized. This can be achieved by mounting said superheater tubesimmediately downstream of the moisture separation system, so that thereis no increase in the temperature of the flow medium flowing therein dueto additional intervening superheater tubes. This consistently minimizestemperature differences as a cause of damage at the interface.

In an advantageous embodiment, the combustion chamber wall of theonce-through steam generator is formed from evaporator tubes and asidewall of the horizontal gas duct is formed from superheater tubes,the superheater tubes adjacent to the combustion chamber being mounteddirectly downstream of the moisture separation system on the flow mediumside. This effectively minimizes the temperature differences at thevertical interface between evaporator tubes of the combustion chamberand superheater tubes of the horizontal gas duct in the case of thetwo-pass boiler.

Advantageously, the top of the once-through steam generator is foundfrom superheater tubes which are disposed immediately downstream of themoisture separation system on the flow medium side. This means that thesuperheater tubes of the top are mounted parallel with other superheatertubes adjacent to the evaporator tubes. Due to the paralleling of theheating surfaces, such an arrangement is advantageous in respect of thepressure loss to be expected.

In a once-through steam generator in which superheater tubes in parallelcontiguity with evaporator tubes are disposed vertically, these areadvantageously designed such that the flow medium flows through thesuperheater tubes from top to bottom. This means that, in the event ofoverfeeding of the moisture separation system resulting in unevaporatedflow medium being applied to the superheater tubes, this can be drainedoff at the outlet header of the superheater tubes, thereby enabling anyflow stagnation to be effectively prevented.

The advantages achieved with the invention are in particular that bymounting superheater tubes in parallel contiguity with evaporator tubesimmediately downstream of the moisture separation system on the flowmedium side, the temperature differences between said tubes areconsistently minimized. As a result, the differential thermal expansionis minimized and damage and overloading are prevented, in turn resultingin fewer repairs and a longer service life of the once-through steamgenerator.

Such an arrangement is advantageous particularly in the case ofonce-through steam generators without circulating pump. The absence ofcirculation results in lower inlet temperatures to the evaporator,smaller steam mass flows and an increase in the firing capacity requiredat startup. Simulations have shown that particularly for these systems,impermissible temperature differences can occur at the interface betweenevaporator and superheater tubes if—as hitherto usual—the superheatertubes at the interface are mounted downstream of other superheatertubes, e.g. of the top. Mounting said superheater tubes directlydownstream of the moisture separation system effectively prevents thesetemperature differences.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention will now be explained ingreater detail with reference to the accompanying drawings in which theFIGURE schematically illustrates a once-through steam generator oftwo-pass design.

DETAILED DESCRIPTION OF INVENTION

The once-through steam generator 1 according to the FIGURE comprises acombustion chamber 2 implemented as a vertical gas duct, downstream ofwhich a horizontal gas duct 6 is disposed in an upper region 4. Thehorizontal gas duct 6 is connected to another vertical gas duct 8.

In the lower region 10 of the combustion chamber 2, a number of burners(not shown in greater detail) are provided which combust liquid or solidfuel in the combustion chamber. The wall 12 of the combustion chamber 2is formed of steam generator tubes welded together in a gas-tight mannerinto which a flow medium—usually water—is pumped by a pump (not shown ingreater detail), said flow medium being heated by the heat produced bythe burners. In the lower region 10 of the combustion chamber 2, thesteam generator tubes can be oriented either spirally or vertically. Inthe case of a spiral arrangement, although comparatively greater designcomplexity is required, the resulting heating differences betweenparallel tubes are comparatively lower than with a vertically tubedcombustion chamber 2.

To improve flue gas flow, the once-through steam generator 1 shown alsocomprises a projection 14 forming a direct transition to the bottom 16of the horizontal gas duct 6 and extending into the combustion chamber2.

The steam generator tubes of the combustion chamber 2 are designed asevaporator tubes. The flow medium is first evaporated therein and fedvia outlet headers 20 to the moisture separation system 22. In themoisture separation system 22, not yet evaporated water is collected anddrained off. This is particularly necessary in startup mode when alarger amount of flow medium must be pumped in to ensure reliablecooling of the evaporator tubes than can be evaporated in one evaporatortube pass. The steam produced is fed to the inlet headers 24 of thedownstream superheater tubes which form the top 26 of the once-throughsteam generator 1 and the walls of the horizontal gas duct 6. Thetransition from the sidewalls of the vertical gas duct to the sidewallsof the horizontal gas duct 6 constitutes the interface 18 betweenevaporator tubes of the combustion chamber wall 12 and superheater tubesin the walls of the horizontal gas duct 6.

In addition to the two-pass boiler shown in the FIGURE, otherconfigurations for fossil fired boilers are self-evidently alsopossible.

In order to prevent damage due to differential thermal expansion causedby temperature differences at the interface 18 between the evaporatortubes of the combustion chamber wall 12 and the superheater tubes in thewalls of the horizontal gas duct 6, these superheater tubes are mounteddirectly downstream of the moisture separation system 22 via aconnecting line 28. As a result, said superheater tubes are only subjectto saturated steam and not higher-temperature superheated steam, therebyreducing the temperature.

The superheater tubes in the walls of the horizontal gas duct 6 areparallel to those of the top 26 and are flowed through from top tobottom. Thus, in the event of overfeeding of the moisture separationsystem 22, unevaporated flow medium in the outlet headers 30 of thesuperheater tubes can be drained off and flow stagnation cannot occur.

The arrangement described minimizes the temperature differences at theinterface 18 between the evaporator tubes of the combustion chamber wall12 and the superheater tubes in the walls of the horizontal gas duct 6,thereby enabling damage to be effectively prevented. This results incomparatively fewer repairs and a longer service life of theonce-through steam generator 1.

1.-4. (canceled)
 5. A continuous steam generator, comprising: aplurality of burners for fossil fuel; a plurality of steam generatortubes; a combustion chamber; a vertical gas duct; a horizontal gas duct;and a moisture separation system, wherein a surrounding wall of theplurality of burners is completely or partially formed from steamgenerator tubes welded together in a gas-tight manner, wherein theplurality of burners are disposed in the combustion chamber downstreamof which the vertical gas duct is mounted above the horizontal gas ducton a hot gas side, wherein a first part of the plurality of steamgenerator tubes is implemented as a system of evaporator tubes mountedupstream of a moisture separation system on a flow medium side, andwherein a second part of the steam generator tubes is implemented as asystem of superheater tubes mounted downstream of the moistureseparation system on the flow medium side, and wherein a plurality ofsuperheater tubes in parallel contiguity with the plurality ofevaporator tubes are mounted immediately downstream of the moistureseparation system on the flow medium side.
 6. The continuous steamgenerator as claimed in claim 5, wherein a combustion chamber wall isformed from the plurality of evaporator tubes and a sidewall of thehorizontal gas duct is formed from the plurality of superheater tubes,and wherein the plurality of superheater tubes adjacent to thecombustion chamber is mounted immediately downstream of the moistureseparation system on the flow medium side.
 7. The continuous steamgenerator as claimed in claim 5, wherein a top of the once-through steamgenerator is formed from the plurality of superheater tubes which aremounted immediately downstream of the moisture separation system on theflow medium side.
 8. The continuous steam generator as claimed in claim5, wherein vertically disposed superheater tubes in parallel contiguitywith the plurality of evaporator tubes are designed such that the flowmedium flows through the plurality of superheater tubes from top tobottom.